An apparatus includes a pulsed-width modulation signal corresponding to a cutoff voltage, a periodic signal generation circuit, a comparator, and logic. The comparator is configured to compare a voltage input with an output of the periodic signal generation circuit and, based on a comparison of the voltage input with the output of the periodic signal generation circuit, generate another pulsed-width modulation signal. The logic is configured to compare the pulsed-width modulation signal and the other pulsed-width modulation signal to determine whether the voltage input has reached the cutoff voltage.